from Amazon
Friday, March 27, 2009
2.5 mph = .025 K per PA = 0.25 ERA
More or less, roughly speaking.
kwERA, which is 5.4 - 12*(K-BB)/PA, would imply a change in ERA of 0.30. Since I like simple numbers, then we can say that 1 mph = .01 K per PA = 0.10 ERA. Going back to Tobin’s paper from yesterday, then his estimate of 0.50 in ERA is bang-on.
When I looked at some pitchers as starters and as relievers, it looks like they gained somewhere from .05 to .10 K per PA. Obviously, they don’t throw 5 to 10mph faster. I can only presume that in addition to the speed gain they have as relievers (say 2-3 mph), they have an equal (or better) gain in movement of pitches, plus of course the advantage of not facing the same batter multiple times.